Ascidian larvae (phylum Chordata, subphylum Urochordata) have a primitive chordate-like central nervous system (CNS) which is thought to be composed of a small number of identifiable and addressable neurons. Furthermore, the nervous system of these marine organisms appears to be predetermined in its cell number, cell position and cell function by strict lineages in the embryo. A detailed investigation of their neuroanatomy seems likely to establish these facts rigorously and to make ascidian larvae useful as a "chordate" model for functional and developmental studies. The project will begin to investigate the neuroanatomy of the Ciona intestinalis larva, structural similarities in the CNS between individuals, and possible premetamorphic changes in the CNS during the swimming larval period. In particular, it will describe the precise origins, pathways, and probable function of those neurons in the visceral ganglion which send axons into the caudal neural tube. These axons, cell bodies, and their connections will be described by serial reconstructions from electron microscopy. Antibodies to tubulins, other neural proteins, and some neurotransmitters will also be used immunocytochemically in wholemount and sections to trace these neurons and their axons and to distinguish between neuronal types and ependymal-glial cells. The caudal neurons are believed to be cholinergic, and this will be further investigated immunocytochemically with antibodies to choline acetyltransferase, and by enhanced histochemical staining for acetylcholinesterase. The main general purpose of this small grant project is to develop and refine techniques which can in turn be used to describe the complete structure of the central and peripheral nervous systems of the Ciona larva.